1. Field of the Invention
The present invention relates to an inkjet printing apparatus.
2. Description of the Related Art
Along with a recent wide spread of OA (office automation) equipment such as a personal computer and a word processor, various printing apparatuses are available for printing information output from such equipment on various printing media. Particularly, an inkjet printing apparatus has the advantages of causing less noise, running at a low cost, and having a compact size and structure relatively easily made to support color printing. For this reason, the inkjet printing apparatus is accepted by users for a wide variety of purposes.
Additionally, the volume per ink droplet used in an inkjet printing apparatus is made as fine as several pl (picoliters) or less so as to meet the recent requirement for higher definition printing. Furthermore, there has appeared an apparatus with a printing head which ejects ink droplets of 1.0 pl or less.
The volume of such a fine ink droplet is equal to that of a mist particle, so that it is difficult to control each ink droplet individually. To put it another way, from view point of higher definition printing, it is preferable to apply ink droplets of, for example, 1.0 pl or less to desired positions on a printing medium with accuracy of μm order; however, it is difficult to achieve a desired accuracy because ink droplets thus ejected are influenced by the surrounding air flow.
This phenomenon is particularly a problem in printing at a higher speed. There is an example of an inkjet printing apparatus having an inkjet printing head (hereinafter, also simply referred to as a printing head) with arranged ejection openings. The inkjet printing apparatus performs printing on a printing medium, while moving the inkjet printing head in main scanning directions which are different from a direction of the ejection-opening arrangement. The main scanning of the printing head and the conveyance of the printing medium (sub scanning) are alternately repeated to perform printing. In such a configuration, it is necessary to move the printing head in the main scanning directions at a high speed in order to increase the printing speed. This printing head movement moves the air so strongly as to disturb the flying of the ejected ink droplets.
Moreover, the single ink droplet is divided into several droplets immediately after the ejection, and thus much finer ink droplets called satellites are formed. These finer ink droplets may either be applied to unintended positions, or may stay floating inside the space of the printing apparatus. Moreover, when ink droplets land on a printing medium, finer ink droplets bounce back from the surface of the printing medium. Such finer ink droplets and satellites (hereinafter, these are referred to as ink mists) stay floating in the air, and eventually are adhered to and accumulated inside the apparatus, resulting in various problems. Specifically, for example, the ink mists make the inside of the printing apparatus unclean, deteriorate proper operations of a movable portion of the printing apparatus by adhering thereto, cause various sensors to malfunction, and also adheres to the surface of a printing medium to make it unclean.
In order to deal with such problems, a method to control ink droplets has been proposed (for example, in Japanese Patent Laid-open No. 5-124187 (1993)) as follows. Specifically, an electric field is generated between a printing head and a printing medium, so that ejected ink droplets are attracted to the printing medium by an electrostatic force. Thereby, the ink droplets are applied to desired positions on the printing medium.
In the meanwhile, inkjet printing apparatuses are extensively used by users in a wide variety of fields, and the purposes of the printing also vary. Accordingly, the users select a variety of conditions (printing conditions). Such printing conditions include, for example, the type of printing medium, print quality, and the like. Specifically, the users sometimes select, as a printing medium, a so-called plain paper as well as glossy paper, matte paper, art paper, synthetic paper, cloth, and the like. Moreover, the users may seek high-definition printing, i.e. high-quality printing, or may seek high-speed printing in which a printing speed has priority over a printing quality.
Under such circumstances, the present inventors have found that a simple application of a technique, as described in Japanese Patent Laid-Open No. 5-124187 (1993), may result in inappropriate printing. This application result will be described below.
In order to perform margin-less printing with an electric field generated between a printing head and a printing medium, the following configuration is given. A platen, which supports the printing medium, formed of a conductive material is disposed to a position facing a surface (ejection face) of the printing head provided with ejection openings. By applying a high positive voltage to the platen, the surface (surface supporting the printing medium) is positively charged. Accordingly, polarization occurs in the printing medium being in contact with the platen 107. The supported surface (bottom surface) of the printing medium is negatively charged, while the opposite surface (top surface) facing to the printing head is positively charged. At this point, when ink droplets are ejected to the printing medium from the printing head having an electric potential of zero, the ink droplets travel to and land on the printing medium. Although the liquid ink droplets ejected from the printing head originally have a momentum in the ejection direction, the ink droplets travel toward the printing medium at an accelerated rate while being attracted to the positively charged top surface of the printing medium.
However, the material and thickness of the printing medium differ depending on its type. Accordingly, on the top surface of the printing medium which has a high permittivity from the bottom surface to the top surface thereof and which loses less electricity inside thereof, the voltage applied to the platen tends to appear without loss in its magnitude. In contrast, in a case of a printing medium having a low permittivity and more internal electric loss, the voltage applied to the platen tends to be reduced. Thus, the effect of generation the electric field between the printing head and the printing medium may not be sufficiently achieved.